1. Field of the Invention
This invention relates generally to mobile wireless communication systems. More particularly, the present invention relates to mobile resonator-slot antennas.
2. Description of the Related Art
Air-to-ground and air-to-sea communication and radio transmission/reception use surface antennas for a variety of requirements such as, for example, military Ultra High Frequency (“UHF”) band (225-400 MHz), LOS, SATCOM, etc. For many years submarine UHF communication with satellites has been accomplished by using wide-band antennas incorporated within an extendable mast. Frequently, raising a mast may compromise the ship's stealth. Furthermore, the original designs may be cumbersome, inefficient and cost prohibitive. Numerous attempts directed to lower power capability of and to improve compactness of wireless systems have been undertaken over a period of time.
As a result, a mast-supporting communication system has been at least partially replaced by a buoyant antenna towed by a submarine. Typically, the existing antenna assemblies are configured to have a rigid cylindrical core wrapped in a conductive material sandwiching a piece of dielectric material that is partially exposed to form a shallow cavity.
One of the first submarine-towed floating resonator-slot antennas providing a foundation for further numerous designs is disclosed in J. C. Lee's paper entitled “A Slender Resonator-Slot UHF Antenna” (M.I.T. Lincoln Laboratory, 1981). This paper discloses a relatively efficient UHF slot antenna extending linearly between its opposite ends and having a straight linear slot backed by a shallow cylindrical cavity of a small diameter, as shown in FIG. 1. While being in a seawater tank and with the antenna slot being kept out of contact with the water surface, the antenna's performance (gain) was satisfactory. When the disclosed resonator-slot antenna was tested at sea, the results were not as good as those produced in the seawater tank.
Among various reasons that may explain lower-than-expected results, the topology of the tested vertically polarized slot-antenna and particularly, the linearly extending slot are rather critical. Conceptually, the tested antenna and its numerous subsequent modifications have been premised on an antenna assembly in which the resonator-slot material stays at the apogee of the hemisphere defined by the floating portion of the assembly. Structurally, as seen in FIG. 1, antenna 2 is configured to have a body of conductive material formed with a slot 8, which is backed by a core 4, and a coaxial feeder 6. Accordingly, any deviation from the ideal position would cause a change in its voltage standing wave ratio, and sufficient shift from the apogee, (i.e. making it parallel to the water), would lead to a de-tuning effect. Once the assembly has reached a position in which the slot deviates at about a 90° from the apogee, the antenna ceases to function, since, as is well known in the art, the electromagnetic waves propagate in a plane extending transversely to the longitudinal direction of the slot. Furthermore, the selection of dielectric materials and the dimensions of the tested antenna may also contribute to unsatisfactory gain characteristics produced by the tested antenna. Since the demand for commercial and military mobile wireless systems is on the rise, it is imperative to develop a simple and reliable structure of a resonator-slot antenna. In particular, as discussed above, the problems confronting a resonator-slot antenna designer are the following: (1) cumbersome antenna topology and (2) antenna efficiency as a function of its orientation.
A need, therefore, exists for a miniature mobile slot antenna configured to be immune to its orientation (or roll) relative to the apogee and to exhibit a satisfactory gain regardless of its position.